Method of and apparatus for increasing uniformity of ionization in material irradiated by cathode rays



Nov. 15, 1955 A. J. GALE 2,724,059

METHOD OF AND APPARATUS FOR INCREASING UNIFORMITY OF IONIZATION INMATERIAL IRRADIATED BY CATI-IODE RAYS Filed Aug. 21, 1952 5 Sheets-Sheet1 FIG. I

INVENTOR: Alfred John Gale BY- agzd, ATTYS- A. J. GALE METHOD OF ANDAPPARATUS FOR INCREASING UNIFORMITY OF IONIZATION IN MATERIAL IRRADIATEDBY CATHODE RAYS Filed Aug. 21, 1952 5 Sheets-Sheet 2 FIG. 4

5 Sheets-Sheet 4 77 [Lu/M Flll'er] 887. wi/lr Auxiliary] lfred John GaleINVENTOR: A BY:

Q ,W ATTYS A. J. GALE IONIZATION IN MATERIAL IRRADIATED BY CATHODE RAYSMETHOD OF AND APPARATUS FOR INCREASING UNIFORMITY OF Nov. 15, 1955 FiledAug. 21, 1952 FIG. 8

A. J. GALE 2,724,659 METHOD OF AND APPARATUS FOR INCREASING UNIFORMITYOF Nov. 15, 1955 IONIZATION IN MATERIAL IRRADIATED BY CATHODE RAYS 5Sheets-Sheet 5 Filed Aug. 21, 1952 Effibiencz 67% [will] Fi/fer] 97%[WI/ll Auxiliary FIGJO Acfual Meu 3 v s q Generator Vo/faqe or Curren/Fae/w .172

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.7! Wyn Meu .0/0l fi'n Meu IBNYVENTOR: Alfred John Gale 7 4 7, ATTY'SUnited States Patent dice 2,724,059 Patented Nov. 15, 1955 METHOD OF ANDAPPARATUS FOR INCREASING UNIFORMITY OF IONIZATION 1N MATERIAL IRRADIATEDBY CATHODE RAYS Alfred J. Gale, Lexington, Mass., assiguor to HighVoltage Engineering Corporation, Cambridge, Mass., at corporation ofMassachus etts Application August 21, 1952, Serial No. 3ll5,633

8 Claims. (Cl. tl-49.5)

This invention relates broadly to the ionization of material by cathoderadiation, and more particularly to a method of and apparatus forincreasing the uniformity of ionization produced in material byirradiation with high-energy electrons, thereby increasing eificiency ofoperation. Still more particularly and specifically stated, theinvention relates to the irradiation of a substance, either packaged ornon-packaged, with at least one main fiux of electrons having arelatively high velocity component in a direction normal to the surfaceof said substance, whereby a maximum ionization level is effected insaid substance a certain distance below said surface, and alsoirradiating said substance with at least one auxiliary fiux of electronshaving a relatively low velocity component in a direction normal to saidsurface, and if desired, with more than one auxiliary flux, so as toincrease the ionization level at said surface in relation to saidmaximum ionization level.

The main flux of electrons may be provided in the practice of myinvention by any cathode-ray generator capable of delivering high-energyelectrons in the energy range between .1 and 20 m. e. v., and theinvention is used with advantage whenever the ionization level producedby said main flux at the surface of the material irradiated is less thanthe maximum ionization level.

in one embodiment of means for practicing my invention, the auxiliaryflux of electrons is provided by an auxiliary generator of lower voltageand current than those of the main installation. The use of auxiliarygenerators is of particular importance in multiple generatorinstallations where spares may be used as auxiliaries without impairingtheir readiness to take over the full duty. Alternatively, a maingenerator operating below performance may continue as an auxiliary untilmaintenance is convenient. The use of auxiliary generators in accordancewith my invention is quite different from ordinary double bombardmenttechniques, for both the voltage and the current of the auxiliarygenerator are preferably markedly less than the voltage and the current,respectively, of the main generator.

In an alternative embodiment of the apparatus of my invention, theauxiliary flux of electrons is etfected or provided by decreasing thenormal component of the velocity of some of the electrons in the mainflux. For example, I may place an absorber or filter between the cathoderay window and the product, or I may simply increase the thickness ofthe window. For some products the outer layers of packaging material mayprovide some or all the absorber thickness required.

Efficiency may be raised either by using optimum thickness filters orauxiliary generators. Furthermore. the use of filters or auxiliarygenerators may give greater uniformity of ionization throughout theproduct and, accordingly, minimize unwanted side etfects.

One object of my invention is to increase the efliciency of operation inthe ionization of material in either a packaged or a non-packagedcondition by cathode radiation.

Another object of my invention is to increase the uniformity ofionization produced in such material by cathode radiation.

Another object 'of my invention is to provide means for filteringcathode rays which are used to ionize such material, so as to increasethe ionization producedby said cathode rays in the surface layers of thematerial irradiated, whereby either efficiency of operation oruniformity of ionization or both may be increased.

Still another object of my invention is to increase the ionization inthe surface layers of material which has been irradiated by a maincathode ray installation, by irradiating such material with cathode raysfrom at least one auxiliary generator operating at lower voltage andcurrent than those of the main installation, whereby either etficiencyof operation or uniformity of ionization or both may be increased.

it is clearly to be understood that more than one auxiliary generatormay be employed in the practice of my invention, and in carrying out themethod thereof.

order that the principle of the invention may be more readilyunderstood, these and other objects of the invention will be bestunderstood by reference to the following description when taken inconnection with the accompanying drawings disclosing different apparatusby which the method may be practiced, while its scope will be moreparticularly pointed out in the appended claims.

in the drawings:

Fig. 1 is a view partly in side elevation and partly in vertical centralsection of one form of acceleration tube for the creation of high-energyelectrons, together with a similar view of the lower end of a secondacceleration tube shown merely for the purpose of illustrating ordinarydouble bombardment technique;

Fig. 2 is a side elevation, but partly in vertical section, of the lowerend of the form of acceleration tube shown in Fig. 1, and illustratesthe use of one form of absorber or filter;

Fig. 3 is a view similar to that of Fig. 2, and illustrates the use of athick cathode-ray Window;

Fig. 3A is a view similar to that of Fig. 2 and illustrates the use ofthe outer layers of packaging material in order to provide the absorberthickness required;

Fig. 4 is a diagram illustrating one arrangement of an auxiliarygenerator in conjunction with a main generator;

Fig. 5 is a graph illustrating the ionization/depth relationship forsingle-pass irradiation;

Fig. 6 is a graph illustrating the ionization/depth relationship fordouble bombardment with cathode rays of equal energy and current, with acentral ionization level of 60%;

Fig. 7 is a graph illustrating the ionization/depth relationship fordouble bombardment with cathode rays of equal energy and current, with acentral ionization level of 70%;

Fig. 8 is a graph illustrating the ionization/depth relationship fordouble bombardment with cathode rays of equal energy and current, with acentral ionization level of Fig. 9 is a graph illustrating theionization/depth relationship for double bombardment with cathode raysof equal energy and current, with a central ionization level of Fig. 10is a graph illustrating the ionization/depth relationship for doublebombardment with cathode rays of equal energy and current, with acentral ionization level of 97 /2%; and

Fig. 10A is a graph similar to that of Fig. 10 and illustrates theionization/depth relationship where a filter is used on the auxiliarygenerator.

The principal effect upon material irradiated thereby, of cathode raysin the energy range between .1 and 20 m. e. v., is ionization. Thisionization may produce various useful results, including sterilizationand deinfestation of foods and drugs, acceleration of chemicalreactions, coloration of glass, and treatment of skin diseases. Whateverthe ultimate object to be achieved, the immediate elfect of the cathoderays is ionization of the material irradiated thereby, and the dosereceived at any point in the material irradiated is proportional to theionization at that point.

The dose required should be and is herein referred to in terms of theminimum dose which must be received at all points within the material.The minimum dose produced by the high-energy electrons must therefore beequal to the minimum dose required. If the ionization produced by thehigh-energy electrons is not uniform, it will be necessary that someportions of the material receive a dose greater than the minimum doserequired. More energy is thus expended than is necessary, andinefficiency of operation results. Furthermore, unwanted side effectsmay be produced in those portions of the material which receive morethan the minimum dose required.

Although my invention may be used in connection with and may bepracticed with the use of various types of cathode ray generators, Iprefer to use a generator of the type shown in the U. S. patent toRobinson, No. 2,602,751, dated July 8, 1952, Serial No. 179,910, andassigned to the same assignee as the assignee of the present invention.With such a generator, the product may be irradiated by scanning anelectron beam of homogeneous energy across the width of the product,while the product moves at constant speed in a direction perpendicularto the direction of scan. In this way the entire surface of the productmay be substantially uniformly irradiated, so that the ionizationproduced at any depth in the product may be substantially uniformthroughout the product at that depth. l However, the ionization is notuniform in depth. The electron beam is unable to penetrate beyond acertain depth, and so no ionization is produced below this point. Forexample, 2 m. e. v. electrons have a maximum penetration of about 1 cm.in water. Maximum ionization is produced at a depth of about /3 themaximum penetration, while the ionization produced at the surface of theproduct is about 60% of maximum ionization.

Greater economy is obtained when double bombardment techniques areadopted, wherein the product is irradiated from opposite sides bycathode rays of equal energy and current. Substantial uniformity ofionization in the central part of the product may also be obtainedthrough the use of double bombardment. However, the ionization producedat the surface of the product is still only 60% of maximum ionization,and this relatively low ionization in the surface layers limits theuniformity and efiiciency obtainable.

Referring to the drawings, and first to Fig. 1 thereof, therein is shownsuch parts of a cathode-ray generator as are necessary to anunderstanding of this invention. The said generator is of the type shownin the patent to Denis M. Robinson, No. 2,602,751, dated July 8, 1952,

Serial No. 179,910, The invention herein claimed may be practiced by thesaid type of cathode-ray generator but my invention is not limited tobeing practiced by or with such type of generator.

In said Fig. l, a small portion, broken away, of one form or type of theacceleration tube of a Van deGraaff electrostatic generator, but to theuse of which my invention is not limited,is indicated at 1. The saidgenerator is capable of producing a narrow beam of high speed electrons,the energy whereof is or may be on the order of several million voltsand as much as five million or more volts, and is manufactured by HighVoltage Engineering Corporation of Cambridge, Massachusetts. The lowerend 2 of said acceleration tube 1 is shown as markedly outwardly flaredor flaring at 3 to left and to right (that is, in two oppositedirections) so as in one direction to be in cross section of greatlyelongated length as indicated, but the width of such flaring portion isabout the same as the normal diameter of said acceleration tube abovethe flaring portion, or it may be less. The said flaring portion 3terminates at its extreme lower end in a window 4, which is a longnarrow slot and is covered by a thin aluminum foil. Such window mustsupport atmospheric pressure on the outside with vacuum on the inside.The narrowness of the slot and the support given by the closely spacedlong sides of the frame insure this.

The length of the said window 4 is preferably such that I may impart tothe electron beam within the said flaring portion 3 a scanning orsweeping motion extending through fifty degrees or more, and if desiredas much as ninety degrees. For that purpose I employ magnetic cells, orI may employ parallel conducting plates, or other suitably shapedelectrodes (not shown), and I preferably make said magnetic coils orconducting plates small enough to position them suitably within theflaring portion 3. This gives close coupling with the electron beam andaccordingly reduces the scanning power required. Also the material ofthe vacuum wall thus serves as a shield against stray external fields.

The high speed electron beam emanating from the cathode of theacceleration tube 1 is indicated at 5. The electrons of the beam 5 areaccelerated through the vacuum region of the acceleration tube 1 in amanner not necessary to explain herein in detail, and they travel in astraight line or path until they reach the flared portion 3, at whichpoint the scanning or sweeping motion referred to above is imparted tothe electron beam;

A conveyor belt 6, or other suitable conveyor, which supports theproduct to be irradiated, indicated diagrarn matically at 7, ispositioned close below the window 4, which is at the extreme lower edgeof the said flaring portion3. Said belt 6 is driven at constant speed ina direction preferably perpendicular to the plane of the drawing, andhence preferably perpendicular to the direction of scan.

Improved operation may be obtained if the product 7 is simultaneouslyirradiated from the opposite side by a second cathode-ray generator, thelower end of which is indicated at 11, and which generator is desirablysubstantially identical with the first generator 1. If the product 7 isin solid mass form, the same effect may be achieved by sequentiallyirradiating the product from opposite sides by the same generator. Suchtype of treatment, either simultaneous or sequential, I call doublebombardment, as opposed to single-pass irradiation wherein or wherebythe product is irradiated from one side only.

The apparatus embodying or for practicing the method of my inventionwill now be described in terms of its application to the firstgenerator 1. If double bombardment is used, it will be understood thatthe same apparatus is to be applied to the second generator 11.

In one embodiment of the apparatus of and for practicing the method ofmy invention, I place an absorber or filter 8 immediately below thewindow 4, as shown in Fig. 2. Said absorber or filter 8 comprises a thinsheet of material which may be attached to the lower extremity of theflared portion 3 in any convenient manner; for example, it may becemented thereto. Said sheet 8 may be of any material but is preferablya metal of low atomic number, in order to minimize back-scatter wherebyelectrons are reflected back towards the acceleration tube. Inparticular, I prefer to use an aluminum absorber, because in addition tohaving a low atomic number, aluminum has a high thermal conductivity, sothat heat generated in the filter by the high-energy electrons isquickly dissipated; Furthermore, aluminum filters are readily formed byrolling and machining. The sheet 8 extends beyond the edges of thewindow 4, so as to cover the window 4 completely and thus intercept anelectron 5 beam 5 at all times. The thickness of sheet 8 is determinedin a manner to be set forth hereinafter, but in general the thicknesswill been the order of 1 mm. for an aluminum sheet.

Instead of attaching a thin sheet of aluminum 8 to the lower extremityof the flared portion 3, I may increase the thickness of the saidaluminum sheet or foil which covers the, window 4. The additionalthickness is computed in the same manner as is computed the thickness ofthe thin sheet 8 shown in Fig. 2. Fig. 3 illustrates the use of athickened window 8a instead of a separate filter. Alternatively, theouter layers of packaging material surrounding the product 7 may providethe required filter thickness 8b, as shownin Fig. 3A. The thickness ofthe packaging material must be such that the required amount ofabsorption is provided. The proper thickness is computed in the samemanner as that employed in computing the thickness of the thin sheet 8illustrated in Fig. 2.

In the second embodiment of the apparatus of my invention, for thepractice of the method thereof, I use an auxiliary generator incombination with each main generator. One possible arrangement is showndiagrammatically in Fig. 4. The auxiliary generator A operates at lowervoltage and current than those of the main generator M.

Referring now to the diagram, Fig. 5, therein is illustrated thevariation of ionization with depth for single pass irradiation by asingle cathode-ray generator without filter. Ionization is expressed asa percentage of maximum ionization. Depth is expressed as a g./cm. m. e.v. or, alternatively as lb./in. -m. e. v. When these units are used, thecurve is substantially independent of the material irradiated andapproximately independent of the energy of the bombarding electrons. Thecurve is plotted for 2 m. e. v. electrons, and the inset curve gives thecorrection for electron energies other than 2 m. e. v. To avoidambiguity, the terms normalized depth and normalized thickness will beused to indicate measurements in g./cm. m. e. v., while the terms lineardepth and linear thickness will refer to measurements in cm. Normalizeddepth is calculated by multiplying the linear depth by the density ofthe material irradiated and dividing the product by the energy of thebombarding electrons.

, Since the acceleration tube by which the method of my invention ispracticed is highly evacuated, practically no ionization takes placewithin the tube, so that normalized depth= is taken as the inner surfaceof the cathode-ray window. The diagram, Fig. 5, shows that theionization at this point is about 55%. If the aluminum-foil cathode-raywindow has a linear thickness of .0074 cm., the normalized thickness ofthe window is .0074 cm. 2.7 g./cm.

where the density of aluminum is taken as 2.7 g. /crn. and where theenergy of the electrons is taken as 2 m. e. v. Since the product ispositioned very close to the window the normalized thickness of theintervening air is negligible. Consequently, that part of the surface ofthe product where the electrons enter the product corresponds to anormalized depth of 0.1 g./cm. -m. e. v. This part of the surface I callthe near surface, shown at n in the diagram Fig. and the correspondingnear surface ionization level is shown by the diagram Fig. 5 to be about60%.

Ionization increases with increasing depth up to a maximum ionizationlevel of 100% at depth m. Beyond this depth m, ionization decreases withincreasing depth. The part of the surface of the product where theelec-. trons emerge from the product I call the remote surface,. shownatr in Fig. 5, and it is apparent from the diagram Fig. 5 that thecorresponding remote surface ionization level depends upon thenormalized thickness of the product.

As hereinbefore-stated, one object of this invention is to increaseuniformity of ionization, and another object is to increase efiiciencyof operation. Uniformity of ionization is defined as the quotient of theminimum ionization level in the product divided by the maximumionization level therein. If at least a certain minimum ionization mustbe effected at all depths in the product, but if greater ionization thanthis minimum is not required, then efficiency of operation may bedefined as follows:

minimum ionization levelX normalized product thickness I where I is thetotal ionization which depends only upon the current and voltage of thegenerator.

For singles-pass irradiation, illustrated in the diagram, Fig. 5, themaximum ionization level is always by definition. Consequently,uniformity is always equal to the minimum ionization level. It isapparent from the shape of the curve in the diagram, Fig. 5, thatuniformity is always equal to either the near surface ionization levelor the remote surface ionization level, whichever is lower. The remotesurface ionization level may be increased by decreasing the normalizedthickness of the product (i. e. by decreasing the linear thickness ofthe product or by increasing the generator voltage). However, the nearsurface ionization level limits the nniformtiy obtainable to 60%.

For a minimum ionization level of 60%, the maximum normalized productthickness obtainable is that which corresponds to a remote surfaceionization level of 60%, and is indicated in the diagram, Fig. 5, by thenormalized thickness of n-r. Said normalized thickness n-r is seen fromthe diagram Fig. 5 to be .33 g./cm. -rn. e. v. If water is beingirradiated with 2 me. v. electrons, the corresponding linear productthickness 18 Efficiency:

Therefore, the maximum efliciency obtainable with the maximum uniformityof 60% is This may be calculated to be 58.5 Minimum ionization levels ofless than 60% result in decreased eificiency,

rays which pass through an aluminum foil window hav-' ing a normalizedthickness of .01 g./cm. -m. e. v. Hence the total normalized depth is YThe diagram Fig. 7 illustrates the irradiation of a product having anormalized thickness of .73 g./cm. m. e. v. by double bombardment withcathode rays which pass through a total normalized absorber thickness of.035 g./cm. -m. e. v. Hence the total normalized depth is The diagramFig. 8 illustrates two cases: (a) the irradiation of a product having anormalized thickness of .76 g./cm. -m. e. v., by double bombardment withcathode rays which pass through an ordinary aluminum foil window, and(b) the irradiation of a product having a normalized thickness of .65g./cm. -m. e. v., by double bombardment with cathode rays which passthrough an increased absorber thickness. Both cases result in the sametotal normalized depth:

(a) .01+.76+.01=.78 g./cm. -m. e. v. (b) .065+.65+.065=78 g./cm. -m. e.v.

Similarly the diagram Fig. 9 illustrates a total normalized depth of .75g./cm. -m. e. v., and Fig. 10, the diagram, a total normalized depth of.73 g./cm. -m. e. v.

For double bombardment without filter and without auxiliary generatorthe two near-surface ionization levels n and n are always 60%, as shownin the diagrams Figs. 6, 8-10. Since there is no remote surface indouble bombardment, the near surface ionization level may be referred tosimply as the surface ionization level. The maximum ionization levels,corresponding to depths m and m, are always at least 100%, although theymay exceed 100% (as shown, for example, in the diagram Fig. 10), sinceionization is expressed as a percentage ofthe maximum single-passionization level.

It is apparent from the shape of the curves in the diagrams Figs. '6-10that the minimum ionization level will be equalto either the surfaceionization level or the central ionization level 0, whichever is thelesser, where the term central ionization level" is defined as thelowest ionization level between depths m and m. The central ionizationlevel may be'increased by decreasing normalized product thickness. Thus,the central ionization level 0 in the diagram Fig. 6 is 60% with anormalized product thickness of .81 g./cm. -m. e. v., whereas thecentral ionization level c in the diagram Fig. is 97 /2% with anormalized product thickness of .71 g./cm. -m. e. v. However, uniformityis limited bythe surface ionization levels n and n' to a maximum Sincetwo identical electron beams are employed in double bombardment, thetotal ionization is'2(I) and efficiency is equal to For a minimumionization level of 60%, the maximum normalized product thicknessobtainable shown at n-n' in the diagram Fig. 6 is .81 g./cm. m. e. v.,so that the maximum efliciency obtainable with the maximum uniformity of60% is This is greater than the maximum efliciency of 60%X.33 g./cm. m.e. v. I

obtainable with single pass irradiation, and may be calculated to be71.5%. Minimum ionization levels of less than 60% result in decreasedefficiency, so that the maximum efficiency obtainable with ordinarydouble bombardment is 71.5%.

To summarize, the maximum uniformity obtainable with ordinarysingle-pass irradiation or with ordinary double bombardment is 60%. Themaximum efiiciency obtainable with ordinary singlepass irradiation is58.5%, and the maximum efficiency obtainable with ordinary doublebombardment is 71.5%. Both uniformity and efficiency are limited by theionization level near the surface of the product.

:.'As stated, in one embodiment of apparatus for practicing the methodof my invention I raise the ionization level near the surface of theproduct by providing-a filter, as shown in Fig. 2. The effect of such afilter is to increase the normalized depth corresponding to the surfaceof the product. Thus, if an ordinary aluminum foil window is used, thesurface of the product corresponds to a normalized depth r'"1=.01g./cm.=-m. e. v., as shown in the diagram Fig. 6. By using an aluminumfoil window of increased thickness, the surface of the product may becaused to correspond to a normalized depth 'f=.035 g./cm m. e. v., asshown in the diagram Fig. 7. The same effect is achieved by placing anabsorber of normalized thickness .025 g./cm. m e. v'. between anordinary thin window of normalized thickness .01 g./cm. m e. v. and theproduct. The diagram, Fig. 7, shows that if the total normalizedabsorber thickness is .035 g./cm. m. e. v., the ionization level at thesurface f is 70%. With 2 m. e. v. electrons the required totalnormalized absorber thickness may be achieved by using an aluminumwindow of linear thickness.

.035 g./cm. m. e. v. X2 in. e. v. 2.7 g./cm. or by using an absorber oflinear thickness By making the normalized product thickness nn' equal to.73 g./cm. -m. e. v., so that the central ionization level c is also70%, a uniformity of 70% and an efliciency of 70%X .365 g./cm. m. e. v.

may be obtained.

The surface ionization level may be further raised simply by increasingthe total normalized absorber thickness, which is equivalent toincreasing the normalized depth 1 of the surface of the product. Thediagram Fig. 8 shows that with a total normalized absorber thickness'of.065 g./cm. m. e. v. the surface corresponds to a normalized depth f of.065 g./cm. m. e. v., and the surface ionization level is Similarly,when the surface corresponds to a normalized depth 1- of .095 g./cm. m.e. v., the surface ionization level is as shown in the diagram Fig. 9;and when the SLll'". face corresponds to a normalized depth of f .13g./cm. m. e. v., the surface ionization level is 97 /2%, as shown in thediagram Fig. 10.

Since the central ionization level may also be increased simply bydecreasing the normalized product thickness n-n', it is possible toincrease uniformity up to almost 100% through the use of filters.Although the use of very thick filters will decrease efiiciency ofoperation, owing to the energy used up by absorption outside theproduct, the use of a filter of moderate thickness actually results ingreater efficiency than that ohtainable with an ordinary thin window.The following table shows that a total normalized absorber thickness of.065 g./cm. m. e. v. results in maximum cfiiciency with doublebombardment.

Total absorber thickness in g./cm. -m. e. v. Uniformity, Efficiency,

(ordinary double bombardment) percent percent The use of filters withsingle-pass irradiation does not the method of my invention, I raise theionization level near the surface of the product by using. an auxiliarygenerator in conjunction with each main enerator, as shown in Fig. 4.The effect of the auxiliary generator upon the ionization level near thesurface of the product is shown for double bombarment in the diagramsFigs. 8-10.

Referring more particularly to the diagram Fig. 8, it will be recalledthat a uniformity of 80% and an efficiency of 77% were obtained byirradiating a product of normalized thickness ff=.65 g./cm. -m. e. v.,with cathode rays passing through a total normalized absorber thickness1 of .065 g./cm. m. e. v. If an ordinary aluminum foil window withoutfilter be employed, the total normalized absorber thickness is reducedto n=.0l g./cm. -m. e. v., and the normalized product thickness isincreased to Uniformity is then reduced to 60%, owing to the ionizationlevel at the surfaces n and n. However, if the product is alsoirradiated by an auxiliary generator of lower voltage and current thanthose of the main generator, the additional ionization produced in thesurface layers nf and nf"may raise the uniformity to 80%.

The ionization/ depth curve for the auxiliary generator is shown at A inthe diagram Fig. 8. This curve A is obtained by transferring theionization/depth curve of the diagram Fig. 5 onto the graphs of Fig. 8with appropriate adjustments for the difference in units. Thus thediagram Fig. 5 shows the ionization/ depth curve for the auxiliarygenerator, where ionization is expressed as a percentage of the maximumionization level produced by the auxiliary generator; but in the diagramFig. 8 ionization is expressed as a percentage of the maximum ionizationlevel produced by the main generator. If the auxiliary generator currentfactor is .23, which means that the current of the auxiliary generatoris 23% of the main generator current, the maximum ionization level ofthe auxiliary generator Will be 23% of the maximum ionization level ofthe main generator, as shown by curve A on the graph of Fig. 8.

Similarly the diagram Fig. 5 shows the ionization/ depth curve for theauxiliary generator, where depth is expressed as @E a th ten it Depth(Flg' 5) Auxiliary generator voltage but in the diagram Fig. 8 depth isexpressed as (Linear de@) (Density) T Main generator voltage Depth (Fig.8)

Auxiliary generato voltage Main generator voltage X (Depth =(Auriliarygenerator voltage factor) X (Depth (Fig. 5))

Thus, for example, the maximum penetration of the auxiliary generator isshown by the diagram Fig. 5 to be .565 g./cm. m. e. v. If the auxiliarygenerator has a voltage factor of .13, the maximum penetration on thegraph of Fig. 8 will correspond to a normalized depth of that with anauxiliary generator voltage factor of .13 and an auxiliary generatorcurrent factor of .23, the ionization level throughout the surfacelayers 71- and rf-f' is raised to at least Uniformity is thus 80%, andthe efiiciency of the main generators is However, the eificiency of themain generators is which may be calculated to be 93.5%.

Similarly the diagram Fig. 10 shows that an auxiliary generator voltagefactor of .27 and an auxiliary generator current factor of .52 willraise the ionization level throughout the surface layers n-f and n'f' toat least 97 /2%. The central ionization level c is also raised 97 /2% byreducing the normalized product thickness n-n to .71 g./cm. m. e. v. Thehigh. ionization level in the surface layers n and n-f limits theuniformity to However, the efiiciency of the main generators is whichmaybe calculated to be 97%.

The efiiciency of the main generators may be increased still further byincreasing the voltage and current of the auxiliary generator. However,the increased voltage and current required of the auxiliary generatorwould become rapidly uneconomical, and the overall efliciency would bereduced. Furthermore, the resultant increased ionization level in thesurface layers would cause a marked decrease in uniformity.

The following table illustrates the variation of uniformity andmain-generator efficiency with auxiliary generator voltage and current.

Auxiliary generator Main genvmform erator efiiciency,

voltage factor current factor cent percent (ordinary double bombardment)2 An auxiliary generator may also be used to raise the efficiency ofsingle-pass irradiation. The remote surface ionization level may beraised by decreasing normalized, product thickness, and the near-surfaceionization level may be raised by using an auxiliary generator. Thediagram Fig. 5 shows that for a normalized product thick ness 019.27g./cm. -m. e. v. the remote surface ionization level will be 80%. It hasbeen hereinbefore stated that in the case of double bombardment, anauxiliary generator of voltage factor .13 and current factor .23' willraise the surface ionization level to 80%, as shown in the diagram, Fig.8. Similarly, the near-surface ionization level in the case ofsingle-pass irradiation may be raised to 80% by the use of suchanauxiliary generator, as shown in the diagram Fig. 5. Uniformity willthen be 80%, and efficiency will be 80% .27 g./cm.-m. e. v. 51

which may be calculated to be 66% Referring again to double bombardmentand in particular to the diagram Fig, 10, therein it is shown that byusing an auxiliary generator it is possible 'toincrease the maingenerator efficiency to 97%. However, this increased efficiency isaccompanied by two disadvantages. First, the high ionization level inthe surface layers n and n'- tends to limit uniformity. Second, thecurrent required of the auxiliary generator is more than half that ofthe main generator. High current may be more difficult to obtain thanhigh voltage, especially if a generator of the type shown in theaforementioned patent to Robinson is employed.

In the diagram Fig. 10, curve A shows the ionization/ depth relationshipfor an auxiliary generator voltage factor of .27 and an auxiliarycurrent factor of .52.

By using a filter on the auxiliary generator, I am able to increaseuniformity in the surface layers n and n'.f'. As hereinbefore set forth,the effect of a filter is to increase the normalized depth correspondingto the surface of the product. If the total normalized absorberthickness on the auxiliary generator is such that the maximum ionizationlevel produced by the auxiliary generator occurs at the surface n of theproduct, the auxiliary generator ionization/depth curve will appear asshown at A in the diagram Fig. a, and the ionization level produced bythe two generators together is shown at B. With such a filter, theauxiliary generator voltage factor required is .38, and the auxiliarygenerator current factor required is .37. It is apparent from acomparison of the diagrams Figs. 10 and 10a that although the additionof the filter increases the voltage factor'from .27 to .38 the currentfactor is reduced from .52 to .37.

Having thus described several embodiments of apparatus for practicingthe method of my invention, it is to be understood that althoughspecific terms are employed, they are used in a generic and descriptivesense and not for purposes of limitation, the scope of the inventionbeing set forth in the following claims.

I claim:

1. Apparatus for irradiating substances with high energy electrons,comprising in combination, a main cathode-ray generator, including meansfor creating a ,main beam of high velocity electrons and also includingmeans for directing said main beam from one aspect onto the surface ofthe susbtance to be irradiated, and an auxiliary cathoderay generator oflower voltage and current than those of said main cathode-ray generator,said cathode-ray generator of lower voltage and current including meansfor creating an auxiliary beam of electrons having a voltage less thanone-half the voltage of the electrons in said main beam, the current insaid auxiliary beam being less than three-fifths the current in saidmain beam, and also including means for directing said auxiliary beamfrom the same aspect onto the surface of such substance.

2. Apparatus for increasing the uniformity of ionization produced in asubstance by the passage of high energy electrons therethrough,comprising a source of high energy electrons, means for directing saidhigh energy electrons onto the substance to be irradiated, and anabsorber positioned between said source and said substance, saidabsorber having a total normalized thickness of more than .01 g./cm. m.e. v. and less than .16 g./cm. -m. e. v.

3. Apparatus for increasing the efliciency of operation in theirradiation of substances by double bombardment with high energyelectrons, comprising at least one source of high energy electrons,means for directing said high energy electrons onto the substance to beirradiated, and an absorber positioned between each of such source andsaid substance, each absorber having a total normalized thickness ofapproximately .065 g./cm. m. e. v., and said substance having a totalnormalized thickness of approximately .65 g./cm. m. e. v.

4. That method of making more nearly uniform throughout all the volumeof a substance, the 'degree of ionization therein that is effected byirradiation with high-energy electrons, which method comprisesirradiating said substance with at least two essentially opposed mainfluxes of electrons, said main fluxes having respective currents suchthat the maximum ionization produced thereby in said substance is notmore than the maximum ionization desired therein, said main fluxeshaving respective voltages such that the ionization produced in saidsubstance is less than the minimum ionization desired therein at the surface layers only of said substance, and also irradiating said substancewith at least two essentially opposed auxiliary fluxes of electrons,said auxiliary fluxes having respective currents less than three-fifthsthe respective, currents in said main fluxes, said auxiliary fluxeshaving respective voltages less than one-half the respective voltages ofsaid main fluxes. V

'5. That method of making more nearly uniform throughout all the volumeof a substance, the degree of ionization therein that is effected byirradiation with highener'gy electrons, which method comprisesirradiating said substance with at least one flux of electrons having arelatively high velocity component in a direction substantially'normalto the surface of said substance, whereby a maximum ionization level iseffected in said substance; and

decreasing the component normal to said surface, of the velocity of someof the electrons in said flux by directing said flux through matterhaving a total normalized thickness of not less than .01 g./cm. -m. e.v. and not more than .16 g./cm. m. e. v. before said electrons impingeon said surface, so as to increase the ionization level at said surfacein relation to said maximum ionization level.

6. Apparatus for increasing the uniformity of ionization produced in asubstance by the passage of high energy electrons therethrough,comprising in combination means for creating and directing a beam ofhigh energy electrons,

said means including an evacuated acceleration tube hav-1 ing anelectron source at one extremity thereof and an electron Window at theopposite extremity thereof; and filter means supported to extend acrossthe path of the electrons issuing from said electron window, the totalnormalized thickness of said electron window and said filter means beingnot less than .01 g./cm. m. e. v. and not more than .16 g./cm. m. e. v.

7. Apparatus in accordance with claim 1, wherein said auxiliary.cathode-ray generator includes an evacuated acceleration tube having anelectron source at one extremity thereof and an electron window at theopposite extremity thereof, said electron window having a normalizedthickness of not less than .01 g./cm. m. e. v. and not more than .16g./cm. m. e. v., said normalized thickness being measured with respectto the voltage of said auxiliary cathode-ray generator. 7

13 14 beam, the current in said auxiliary beam being less thanReferences Cited in the file of this patent three-fifths the current insaid main beam, and also includ- UNITED STATES PATENTS ing means fordirecting said auxiliary beam from the same 90 5 aspect onto the surfaceof such substance; and filter means i 33 gg -ggs supported to extendacross the path of the electrons mu 5 2,602,751 Robinson y 2 ing fromsaid auxiliary cathode-ray generator.

